Gases are classified according to carbon isotopic composition of methane.
What is the difference between propane and butane?
In terms of occurrence, natural gas is classified as:
non-associated gas, and
Non-associated natural gas is found in reservoirs in which there is no or, at best, minimum amounts of crude oil. Non-associated gas is usually richer in methane but is markedly leaner in terms of the higher paraffinic hydrocarbons and condensate material.
Non-associated gas, unlike associated gas can be kept underground as long as required. This is therefore discretionary gas which can be tapped when economically and technologically advantageous.
Natural gas found in crude oil reservoirs and produced during the production of crude oil is called associated gas. It exists as a free gas (gas cap) in contact with the crude petroleum and also as a ‘dissolved natural gas’ in the crude oil.
Associated gas is usually contains less methane than the non-associated gas but is richer in the higher molecular weight hydrocarbons. Non-associated gas can be produced at higher pressures whereas associated gas (free or dissolved gas) must be separated from petroleum at lower separator pressures, which usually involves increased expenditure for compression.